/*=========================================================================
 *
 *  Copyright UMC Utrecht and contributors
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0.txt
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *=========================================================================*/
#ifndef __itkBSplineSecondOrderDerivativeKernelFunction2_h
#define __itkBSplineSecondOrderDerivativeKernelFunction2_h

#include "itkKernelFunctionBase.h"
#include "vnl/vnl_math.h"
#include <cmath> // For abs.

namespace itk
{

/** \class BSplineSecondOrderDerivativeKernelFunction2
 * \brief Derivative of a B-spline kernel used for density estimation and
 *  nonparametric regression.
 *
 * This class encapsulates the derivative of a B-spline kernel for
 * density estimation or nonparametric regression.
 * See documentation for KernelFunction for more details.
 *
 * This class is templated over the spline order.
 * \warning Evaluate is only implemented for spline order 1 to 4
 *
 * \sa KernelFunction
 *
 * \ingroup Functions
 */
template< unsigned int VSplineOrder = 3 >
class BSplineSecondOrderDerivativeKernelFunction2 : public KernelFunctionBase< double >
{
public:

  /** Standard class typedefs. */
  typedef BSplineSecondOrderDerivativeKernelFunction2 Self;
  typedef KernelFunctionBase< double >                Superclass;
  typedef SmartPointer< Self >                        Pointer;

  /** Method for creation through the object factory. */
  itkNewMacro( Self );

  /** Run-time type information (and related methods). */
  itkTypeMacro( BSplineSecondOrderDerivativeKernelFunction2, KernelFunctionBase );

  /** Enum of for spline order. */
  itkStaticConstMacro( SplineOrder, unsigned int, VSplineOrder );

  /** Evaluate the function. */
  inline double Evaluate( const double & u ) const override
  {
    return this->Evaluate( Dispatch< VSplineOrder >(), u );
  }


  /** Evaluate the function. */
  inline void Evaluate( const double & u, double * weights ) const
  {
    this->Evaluate( Dispatch< VSplineOrder >(), u, weights );
  }


protected:

  BSplineSecondOrderDerivativeKernelFunction2(){}
  ~BSplineSecondOrderDerivativeKernelFunction2() override{}

  void PrintSelf( std::ostream & os, Indent indent ) const override
  {
    Superclass::PrintSelf( os, indent );
    os << indent  << "Spline Order: " << SplineOrder << std::endl;
  }


private:

  BSplineSecondOrderDerivativeKernelFunction2( const Self & ); // purposely not implemented
  void operator=( const Self & );                              // purposely not implemented

  /** Structures to control overloaded versions of Evaluate */
  struct DispatchBase {};
  template< unsigned int >
  struct Dispatch : DispatchBase {};

  /** Zeroth order spline. */
  // Second order derivative not defined.

  /** First order spline */
  // Second order derivative not defined.

  /** Second order spline. */
  inline double Evaluate( const Dispatch< 2 > &, const double & u ) const
  {
    double absValue = std::abs( u );

    if( absValue < 0.5 )
    {
      return -2.0;
    }
    else if( absValue == 0.5 )
    {
      return -0.5;
    }
    else if( absValue < 1.5 )
    {
      return 1.0;
    }
    else if( absValue == 1.5 )
    {
      return 0.5;
    }
    else
    {
      return 0.0;
    }
  }


  inline void Evaluate( const Dispatch< 2 > &, const double & u, double * weights ) const
  {
    weights[ 0 ] = 1.0;
    weights[ 1 ] = -2.0;
    weights[ 2 ] = 1.0;
  }


  /**  Third order spline. */
  inline double Evaluate( const Dispatch< 3 > &, const double & u ) const
  {
    const double absValue = std::abs( u );

    if( absValue < 1.0 )
    {
      return vnl_math::sgn0( u ) * ( 3.0 * u ) - 2.0;
    }
    else if( absValue < 2.0 )
    {
      return -vnl_math::sgn( u ) * u + 2.0;
    }
    else
    {
      return 0.0;
    }
  }


  inline void Evaluate( const Dispatch< 3 > &, const double & u, double * weights ) const
  {
    weights[ 0 ] = -u + 2.0;
    weights[ 1 ] = 3.0 * u - 5.0;
    weights[ 2 ] = -3.0 * u + 4.0;
    weights[ 3 ] = u - 1.0;
  }


  /** Unimplemented spline order */
  inline double Evaluate( const DispatchBase &, const double & ) const
  {
    itkExceptionMacro( "Evaluate not implemented for spline order " << SplineOrder );
    return 0.0; // This is to avoid compiler warning about missing
    // return statement.  It should never be evaluated.
  }


  inline void Evaluate( const DispatchBase &, const double &, double * ) const
  {
    itkExceptionMacro( "Evaluate not implemented for spline order " << SplineOrder );
  }


};

} // end namespace itk

#endif
